Timing circuits



I June 23, 1959 B. J. NORRIS TIMING CIRCUITS Filed Nov. 25, 1955 PULSE SOURCE I I l I I I I I I I I I I I I I I PULSE AMR SWEEP 2o MULTIVIBRATOR CIRCUIT INPUT PULSES Q'- PULSES FROM PULSE SOURCE b 5 T2 Ta TIMING IF, if 1: I: KPOTENTIAL AT GRID WAVEFORM 1 0F TUBE "1 AT WHICH THE CIRCUIT REGENERATES OUTPUT VOLTAGE INVENTOR.

BEVITT J. NORRIS A T TORNE Y United States Patent TIMING CIRCUITS Application November 195 S erial No. 549,010

1 Claim. (Cl. 250-27) This invention relates to timing circuits of the type employing multivibrators and is particularly directed to means for reducing to zero the recovery time of the multivibrator.

Single shot or monostable multivibrators are convenient relays where a short low energy pulse must operate the relay. Two triodes cross-coupled, with the anode of one coupled to the grid of the other, are convenient for obtaining adjustable lengths of on and off periods. The initiation of an on-and-ofi cycle may be started by a short pulse and the shift to the opposite state follows immediately. Unfortunately, the return to the stable state at the conclusion of the unstable period requires an appreciable time. The recovery must be fully effected before the full sensitivity of the circuit is regained. Thus the recovery time of the conventional single shot multivibrator must be subtracted from the useful duty cycle of the applied pulse.

An object of this invention is means for reducing to zero the recovery time of a multivibrator.

Another object of this invention is to place the on and off shifts of the multivibrator under the control of a sweep circuit which sweep circuit is in turn con trolled by the pulse source.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

Fig. l is a circuit diagram of said one embodiment; and

Fig. 2 illustrates circuit of Fig. 1.

The particular multivibrator chosen for illustrating this vention is of the so-called Schmitt type and comprises two triodes, 1 and 2. The anode of tube 1 is coupled to the grid of tube 2 through the voltage divider consisting of resistance 3 with the parallel coupling condenser 4 and resistor 5. Cathode resistor 6 is common to the cathode circuits of the two tubes 1 and 2. When the input potential applied to the grid of tube 1 is zero, maximum current flows in the second tube, and the anode current of the first tube is cut oil? by the potential drop across cathode resistance 6. If, now, the potential on the grid of tube 1 is increased by an amount which is sufficient to overcome the potential drop across resistance 6 and to permit some anode current to flow in tube 1, the grid potential of tube 2 will become sufiiciently negative to reduce the anode current of tube 2 considerably, thus reducing the value of the potential across resistance 6, which in turn will increase the grid to cathode potential of tube 1. This eifect is cumulative and a rapid change from one state to the other results. Nevertheless, the changeover time is finite and the condenser 4, usually of small value, is usually added across resistance 3 to increase speed of the trigger action. The potential at the anode of tube graphs of important voltages in the 2 which may be employed as the.

output terminal of the multivibrator will assume one of two distinct values, the duration of each value depending upon the duration of the signal at the grid of tube 1.

Such a multivibrator type relay is useful when con nected to a pulse source 10. The pulse source 10 contemplated here is a source of pulses of short duration and of either regular or random reoccurrence, as desired. According to an important feature of this invention, a sweep circuit 20 is connected in the control circuit of the multivibrator, the control circuit being the grid of tube 1 in the example illustrated. The sweep circuit 20 is adapted to drive the control grid of tube 1 immediately to a nonconducting state upon receipt of the pulse from source 10. However, the return to a conducting state of the control grid voltage is delayed by an amount depending upon the slope of the ramp of the sweep portion of the wave produced in the sweep circuit 20. That is to say, if the quiescent state of the multivibrator is disturbed by a pulse from source 10 at time t referring to the voltage V of Fig. 2, rise of voltage V from some fixed low value, and return of the grid to its quiescent state, can positively measure the on period of the multivibrator. By changing the charging time constant as suggested at a, b, c, the on period can be changed. It will be noted that if the charging period is increased beyond the duty cycle, or the reoccurring period of the pulse between times t and t the multivibrator is prevented from returning to its quiescent or stable state. In the latter case, the output potential at the output terminal will remain at a fixed on value throughout the period of reoccurrence of the pulses at times t t 1 etc.

A convenient sweep circuit generator may comprise a simple RC network. Resistance 21 and condenser 22 are connected in parallel with respect to the grid of tube 1. Resistance 21 comprises the load impedance for amplifier 30 for the pulses of source 10. Preferably, the impedance of amplifier 30 is relatively low so that large current will flow to discharge condenser 22 upon receipt of the pulse at the control grid of tube 30, and reduce the anode voltage of pentode 30 to a fixed, low, bottomed voltage. When tube 30 is cut oil? after the application of the short pulse to its control grid, the charge on condenser 22 is restored slowly through resistor 21 at a rate determined by the absolute values of elements 21 and 22. It will be noted, however, that the control grid of tube 1 of the multivibrator follows the sweep of voltage appearing across condenser 22 and is subject to this potential solely. Hence the sweep circuit may control the movement of the voltage of the grid of tube 1 across its cutoif value and therefore controls both the on and off states of the multivibrator.

By adjusting the time constant of the RC sweep circuit, as by an adjustable resistance at 21, or an adjustable condenser 22, the proportion of the on and oil periods of the multivibrator is easily controlled.

While I have described above the principles of my invention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

A circuit of the character described, comprising: a Schmitt-type multivibrator having two amplifiers, each having anode, cathode, and control elements, said cathodes being connected together and to a common biasing resistor, a coupling resistor connected between the anode element of one amplifier and the control element of the other amplifier, an output circuit connected to the anode element of said other amplifier, said one amplifier being normally conducting and said other amplifier being normally cut-off; a third amplifier having anode, cathode Patented June 23, 1959' u and control elements, said anode of said third amplifier being directly connected to the control grid of said one amplifier, said third amplifier being normally cut-off and having its control element connected to a source of positive-going pulses whereby said third amplifier is driven into conduction; and a capacitor connected between the anode and cathode elements of said third amplifier and another resistor connecting the anode of said third amplifier to a source of positive potential, said capacitor and other resistor constituting a time constant circuit, said capacitor being normally charged through said other resistor and being discharged by conduction of said third amplifier thereby cutting-01f said one amplifier and causing conduction of said other amplifier to initiate a negative-going output pulse in said output circuit, recharging the said capacitor determining the duration of said output pulse.

References Cited in the file of this patent UNITED STATES PATENTS 

